Solid Electrolyte Interphase 20

uElectrolyte

As discussed in previous sections, the electrolyte in lithium-ion cell acts as an ionic conductor for lithium transport between anode and cathode during cell cycles. Lithium-ion battery generally adopts lithium aprotic compound as lithium ion source, such as lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium Bis(Oxalato)Borate (LiBOB), etc. The lithium compound is dissolved in alkyl carbonate solvent or solvent mixtures including ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. (Perla B. Balbuena 2004). Several commonly used carbonate solvent structures are presented in Figure 5.

Figure 5 Alkyl carbonate solvent structures

Table 4 Oxidation and reduction potentials of selective alkyl carbonate solvent

One of the most important solvent selection criteria is the reduction and oxidation potential window. On one hand, it is essential for electrolyte to have oxidation potential greater than cathode charge and discharge potential to prevent detrimental oxidation reactions during cell cycles, as well as to maintain thermal stability and ionic conduction; while on the other, electrolyte should be reduced prior to lithiation to form a passivation film on anode surfaces. Selective alkyl carbonate solvent reduction and oxidation potentials with 1M LiPF6 are listed in

Solvent Oxidation Potential /V

(with 1M/L LiPF6) Reduction Potential /V (with 1M/L LiPF6) EC >6 1.36 PC >6 1-1.6 DMC >6 1.32 DEC >6 1.32

Table 1 (Scrosati 2002). Typical charge and discharge capacities versus electrode potential for both anode and cathode in lithium-ion batteries are shown in Figure 6. The dashed lines represent oxidation and reduction potential for electrolytes.

Figure 6 Anode and cathode normalized capacities versus electrode potentials

Anode surface reactions involve carbonate solvent, lithium salt, contaminants such as water, dissolved oxygen, and carbon dioxide in electrolyte under anodic polarization. Reactions products on graphite anodes have been well identified and mechanisms have been discussed extensively in previous studies (Scrosati 2002).

As shown in Figure 7, Aurbach et al. have proposed possible mechanisms for EC reduction to form passivation film on graphite anodes (Aurbach et al. 1999). EC and other organic solvents are reduced and react with lithium ion under cathodic bias through several steps

to generate a continuous passivating organic film on carbon anodes. Reactions of organic solvents, lithium hexafluorophosphate and their derivatives on carbon anodes are summarized as follows (Scrosati 2002):

Figure 7 Various EC reduction patterns on graphite anode surface and relevant products (Reprinted from On the correlation between surface chemistry and performance of graphite negative electrodes for Li ion batteries by Aurbach et al., copyright 1999, with permission from

Elsevier) Alkyl carbonate reduction reactions:

2EC + 2e- _{+ 2Li}+ _{→ CH}

2═CH2 + (CH2OCO2Li)2

2PC + 2e- + 2Li+ → CH3CH═CH2 + CH3CH(OCO2Li)CH2OCO2Li DMC + e- + Li+ → CH3OCO2Li + CH3• or CH3OLi + CH3OCO•

DEC + e- + Li+ → CH3CH2OCO2Li + CH3CH2OLi

Possible reaction with reduction products and contaminant water

2ROCO2Li + H2O → Li2CO3 + 2ROH + CO2

R• + Li → RLi R• + R’• → RR’

Possible surface reaction of lithium salt, eg. LiPF6 with trace amount of water contaminant: LiPF6 ↔ LiF + PF5

PF5 + H2O → POF3 + 2HF POF3 + 2xLi+ +2xe- → xLiF(s) + LixPOF3-x PF5 + 2xLi+ +2xe- → xLiF(s) + LixPF5-x

(CH2OCO2Li)2 + H2O → Li2CO3 + CO2+ (CH2OH)2

ROCO2Li(s) + HF → LiF(s) + ROCO2H

Possible CO2 reduction: CO2 + e- + Li+ → •CO2Li

•CO2Li + CO2 → O═ •CO-CO2Li

O═ •CO-CO2Li + e- + Li+ → CO + Li2CO3 2LiOH + CO2 → Li2CO3 + H2O

Li2O + CO2 → Li2CO3 ROLi + CO2 → ROCO2Li

where R represents alkyl functional groups such as CH3, CH3CH2, etc. uSolid Electrolyte Interphase (SEI)

The SEI is defined as a thin layer (30-50 nm) composed of inorganic and organic products deposited on the anode surface during charge and discharge cycles due to electrolyte reduction and other surface reactions.

Figure 8 Schematic of the SEI in liquid and polymer electrolyte on carbon or lithium anode surfaces (Reprint from Advanced Model for Solid Electrolyte Interphase Electrodes in Liquid

and Polymer Electrolytes by E. Peled et al., copyright 1997, with permission from Electrochemical Society)

A stable and continuous SEI layer is considered as a crucial factor as it provides a protective passivation layer and allows ion conduction for lithium insertion and extraction as well as maintains anode integrity. The SEI on graphite anodes has been extensively studied, including its formation mechanism, composition, morphology and other properties in previous works (Perla B. Balbuena 2004). The SEI has been characterized via XPS, FT-IR, Raman Spectrum and spectroscopic analysis tools. Based on these analyses, the SEI is mainly composed of lithium oxides, lithium salts, and other carbonates. A schematic of the SEI on a graphite or lithium metal anodes is shown in Figure 8 as proposed by Peled et al. (Peled et al. 1997). Lithium oxide, lithium fluoride, and lithium carbonates are attached close to anode surface

within SEI layer, while other organic compounds such as polyolefins and semicarbonates are near electrolyte phase. The SEI is formed in the first few cycles; however, the composition of SEI is dynamic and varies at different anode potentials. The SEI can be formed and dissolved into electrolyte continuously over prolonged cycles (Bryngelsson et al. 2007).

uSEI on Silicon Anodes

SEI layer on silicon anodes is significantly different from the film typically formed on graphite negative electrodes for two main reasons:

(1) The silicon surface is more reactive to electrolytes than graphite and will result in a complex SEI composition that includes hydrocarbons, C2H5OCOOLi, LiCO3, Li2O, LiF, and silicon containing products (such as lithium silicates, SiF62-, etc.) (Chan et al. 2009).

(2) Over 300 % volume change for silicon during lithium insertion and extraction may cause breakage of the SEI and expose reactive silicon surface to electrolytes for further undesired reactions. (Kong et al. 2001; Kasavajjula et al. 2007).

Besides similar electrolyte reduction reactions as summarized for carbon anodes, several specific reactions for silicon anodes were proposed based on the SEI composition reported in previous studies:

SiOx+ 4HF + 2F- + 2h+ → SiF62- + 2H+ + H2O SiOx + 2xLi+ + 2xe- → xLi2O + Si SiO- + 2Li → Li2O + Si

SiOx + Li+ → LiSOx

EC + Li+ + Si + e- → ROCO2Li/Si + other carbonates

Silicon-specific irreversible reactions may result in the consumption of lithium, silicon etching, or products that block lithium transport, leading to further capacity fade.(Kong et al. 2001; Kasavajjula et al. 2007) An in situ analysis has shown that oxidized silicon species may

strip fluoride from complexes such as PF6-, which results in formation of silicon fluorides. These reactions may either result in localized silicon etching, or increase production of LiF or other fluorinated species (Flake et al. 1999). In situ oxidation of silicon anodes and irreversible reactions with fluorine at the silicon-SEI interface (including the formation of organic phosphorus-fluorine and P-F-containing inorganic species) are also known to affect capacity fade in silicon thin film anodes (Song et al. 2009). It has been reported that silicon is reactive with electrolyte decomposition products, such as HF derived from LiPF6 in the presence of trace amounts of water, and forms complex products within the SEI. Such reactions may exacerbate capacity decrease by consuming active silicon in the anode and lithium ions in the electrolyte through prolonged cell cycles (Choi et al. 2007; Song et al. 2009; Yen et al. 2009). Possible SEI composition on silicon anodes is illustrated in Figure 9 based on literature.

Figure 9 Schematic of the solid electrolyte interphase (SEI) on silicon anodes

Various approaches have been applied to mitigate the silicon surface reactions. Among these approaches, coating silicon with conductive carbon or other active/inactive materials is

effective in both protecting silicon as well as increasing internal conductivity within anode matrix, and greatly improves capacity retention for silicon composite anodes (Kim et al. 2008; Cui et al. 2009; Huang et al. 2009; Yu et al. 2009; Xiao et al. 2010).

Electrolyte can be modified with additives to stabilize SEI as well as improve anode performance. Choi et al. have demonstrated that 3 % fluoroethylene carbonate (FEC) additive is effective in creating SEI consisting of stable Si-F and Li-F compounds, and increases reversible discharge capacity up to 88.5 % compared to FEC free electrolyte (Choi et al. 2006). More recently, Ryu and Song have proposed that adding alkoxy silanes in the electrolytes with thin film silicon anodes passivates silicon surface and capacities may be greatly improved.(Ryu et al. 2008; Song et al. 2009) Song et al. showed that reactive binding agents may double the silicon thin film anode capacity (up to approximately 50 % of silicon’s theoretical maximum specific capacity) and significantly improves capacity retention. This result was attributed to Si-O-Si bonds which was proven to form within the SEI, and the Si-O-Si bonds were considered to stabilize the SEI layer (Song et al. 2009; Nguyen et al. 2010).

Surface chemistry of anodes affects the SEI formation and anode performance significantly. Aurbach et al. have studied the effects of surface chemistry on graphite anodes of lithium-ion battery and the SEI on graphite anodes (Aurbach et al. 1999). Although silicon surfaces are considered more reactive than graphite, the relationships between surface chemistry and anode performance are not well established. Chen et al. have described hydride-terminated silicon surfaces as “highly reactive” and shown nanowires with native oxides surfaces with over 50 % increase in capacity retention then hydride-terminated anodes, as well as a significantly different SEI composition (Chan et al. 2009). In addition to influencing anode stability, surface chemistry may also affect the transport and adsorption of lithium. Theoretical calculations based on density function theory show the lithium fast transport is limited by a high intrinsic energy

barrier of lithium surface intercalation for silicon thin film anode, and the high energy barrier can be reduced by surface modification via doping with aluminum(Peng et al. 2010). An ab initio study also shows lithium binding energy is the highest on silicon surface; and Si [110] surfaces are relatively favorable for lithium doping (Zhang et al. 2010). A recent simulation study by Chan et al. also shows silicon without any surface termination has significantly lower binding energy with lithium than silicon with hydride surfaces (Maria et al. 2010).

The SEI is of great importance in silicon anode capacity retention, and more efforts are required to further explore the formation mechanism, composition and properties of the SEI, so as to maintain high reversible anode capacities over prolonged cycles.